For more then a year, my machine has been happily processing SETI work on the GPU. In the last 2 weeks or so I am no longer receiving work for the GPU. The BOINC manager tells me that the GPU processing is enabled and I've tried messing with the setting to no avail.

I have a Windows 7 AMD machine with an AMD Radeon HD 6450 graphics adapter. It's been working OK for a long time - I can't figure out what happened.

As you can see below, the graphics card is found and considered ready to use.

Which do you call "older GPU"? The Radeon HD 7850/7870 ??
Where did you get the driver? Windows update (= wrong)?

Edit:
Why you don't say explicitly you use Linux?
Less people are familiar with Linux (I am not) so make sure OpenCL part of the driver is present in the driver package you used (and you installed/checked the OpenCL part).
And find a Linux tool (clinfo ?) that can check for the presence of OpenCL

I'm suspicious that BOINC is asking SETI@Home for ATI work for this system, but doesn't recognize the OpenCL capability, where as, on my nVidia system, BOINC recognized it as both a CUDA and OpenCL capable GPU but gets openCL work (specifically) from SETI.

If SETI@Home doesn't have CAL code, then that explains it, I think. I just have to figure out why my GPU isn't identified as OpenCL. I swear I checked this every carefully, but now I cannot find the web-page I remember reading. I'm so close, just can't get over this last hump.

I meant the 7800-series is two platforms old in the AMD GPU lineage. Both the R7 and R9 are newer GPU core-technologies. Reading a bit more, I'm not sure that the R7 is an entirely new core, separate from both the 7800s and R9, so maybe it shouldn't count. In anycase, that's what I meant by 'older' in my earlier post.

I have the very latest AMD Catalyst drivers and they work very well for all 3D acceleration. Native Linux games work and even 32-bit Windows 3D games work via WINE, again. I fact, I have the beta drivers, v13.11, since the latest stable Catalyst for Linux is 13.4 and I read there were problems with it. v13.9 is not available and/or not stable (for Linux); it's not clear.

This is my best lead right now: a missing OpenCL runtime implementation.

See here: http://boincfaq.mundayweb.com/index.php?language=1&view=506&sessionID=17915902ebc5eb011cbeb213d548f211

Relevant Excerpt:
You will need to install this SDK as AMD does not allow that projects distribute the needed libraries on their own yet.

You may have noticed a library file for nVidia GPUs that comes with BOINC (or maybe it's from the projects, I can't remember). I think this is what is missing for ATi on Linux. I will try installing the SDK soon and report back as I cannot find a runtime-only pkg.

I'm not certain this will work for several reasons, though. Mainly, I see both GPU & *CPU* OpenCL devices in clinfo, now, which makes me thing everything necessary is actually there. Plus, I see people complaining that OpencCL runtime is not included in Linux drivers, but then AMD promised it was working on it years ago. I would be great of someone could clarify.

I meant the 7800-series is two platforms old in the AMD GPU lineage. Both the R7 and R9 are newer GPU core-technologies. Reading a bit more, I'm not sure that the R7 is an entirely new core, separate from both the 7800s and R9, so maybe it shouldn't count. In anycase, that's what I meant by 'older' in my earlier post.

I can't shed any light on your problem for you, but I thought I'd correct a slightly insignificant fact that the R7 and R9 are both of the same family, meant as a mid-range and high-end offering of the same platform. This makes your 7800-series akin to the 7900-series and only one platform old being of the directly preceding generation of AMD GPU chips.

...and from what I've read, the R-series would be a "tock" in Intel's "tick tock" cadence. Or, put another way: simply a refinement of the 7xxx series GPUs.

I have no idea how that helped, but it did (I think). I installed the AMD APP SDK and now BOINC detects a CAL and an OpenCL GPU resource.

Finally! I have GPU work units!

Frustratingly, the installation would not install components for OpenCL CPU resources because I had the Catalyst driver installed. No explanation is given. I can't believe the state of ATi Linux drivers; it's a nightmare.